While it is not possible to cap employee salaries, each post can be set up to have a range of a lower to upper payscale. When adding an employee to a post, you will only be able to select a payscale that is within this range.
Go to Admin Menu, then Post Setup.
Search for the post and click it.
Click Edit in the Pay Details area.
Select the negotiating body and the lower to upper grades.
Select the lower to upper payscales from the Lower Pay Scale and Upper Pay Scale dropdowns.
Click Save.